The present invention relates to a projection type liquid crystal display device for projecting an image which is formed in a liquid crystal display element on a screen by illuminating the liquid crystal display element with light which is emitted from a light source and, particularly, to an optical system in the projection type liquid crystal display device.
One of the important factors for wide-spread use of projection type liquid crystal display devices, which are suitably used for a large screen image display, is a reduction in cost.
The most effective measure for the reduction in cost is miniaturization of a liquid crystal display element and miniaturization of a projection optical system involved therein. At the same time, a high definition display having a high pixel density such as that achieved by a high quality television is in demand. A reflection type liquid crystal display element is characterized in that it maintains, even if it is a small and high-definition element, an aperture ratio higher than that of a transmission type liquid crystal display element by covering almost all of pixels with electrodes which are also used as reflection plates. Therefore, it can be said that the reflection type liquid crystal display element is the most ideal element for a small and high-definition projection type liquid crystal display device. Examples of the prior art of the projection optical system for the projection type liquid crystal display device using the reflection type liquid crystal display element include Japanese Patent Laid-open No. 8-271854; Japanese Patent Laid-open No. 10-20273; U.S. Pat. No. 6,183,091; WO99/19758; P. M. Alt, Conference Record of the 1997 International Display Research Conference, M19-M28 (1997); R. L. Melcher, SID 98 Digest, pp. 25–28 (1998) and so forth. In particular, the projection optical system disclosed in U.S. Pat. No. 6,183,091 is characterized by a wavelength separation which is performed by effectively using a retarder stack and a polarizing beamsplitter. The retarder stack means an optical element formed by laminating a multiple of phase difference films having reflective index anisotropy properties, the retarder stack having a function of varying a phase difference of a light component selected from three color light components by π with respect to another one of the three color light components. Details of the retarder stack can be found in U.S. Pat. No. 5,751,384.
The projection optical system disclosed in U.S. Pat. No. 6,183,091, however, has the following problems. The projection optical system, which comprises a separation/synthesis system for three color light components having a polarizing beamsplitter, an input retarder stack disposed adjacent to a first sideface of the polarizing beamsplitter, a spacer disposed adjacent to a second sideface of the polarizing beamsplitter, a dichroic beamsplitter disposed adjacent to a third sideface of the polarizing beamsplitter and an output retarder stack disposed adjacent to a fourth sideface of the polarizing beamsplitter, cannot achieve a high contrast ratio since an optical leakage occurs in boundary wavelength regions of the three color light components when performing a black display. Further, the projection type liquid crystal display device using the projection optical system has a problem of deterioration in color purity which is caused by a large overlap portion of different colors on the boundary wavelength regions of the three color light components.
Although a prevailed aspect ratio of screens of personal computer monitors or NTSC televisions has been 3:4, a screen used in an application whose main object is image viewing, such as a screen of a high quality television, is in an oblong shape having an aspect ratio of 9:16. Accordingly, a shape of a liquid crystal panel used in such projection type liquid crystal display device is also oblong, leading to a problem of an increase in size of the color-separation/synthesis optical system which is provided in the projection type liquid crystal display.